LONDON (Reuters) - Oil and natural gas flows along Britain’s biggest and most important oil pipeline, Forties, were ramping up slowly on Thursday as the system restarts after its second unplanned outage in two months, industry sources said.

The pipeline, whose oil underpins the Brent oil pricing benchmark, was shut on Wednesday, causing a brief spike in oil prices and raising concern about the reliability of ageing North Sea infrastructure.

A spokesman for Ineos, the pipeline operator, said on Thursday oil and gas had begun flowing again. The shutdown was caused by the closing of a feed control valve at the company’s Kinneil gas processing plant, Ineos said.

Output at Buzzard, the largest oilfield connected to Forties, has restarted and is about 95,000 barrels per day (bpd), less than the normal rate of around 150,000 bpd, due to a restriction, a trade source said.

A second source said the restriction was related to the Kinneil plant and would be temporary.

“The pipeline needs to ramp up slowly,” the second source said, adding that in the past it had taken one or two days for normal flow rates to be reached.

Nexen, operator of the Buzzard field, could not be reached for immediate comment on its output, while Ineos could not be reached for immediate comment on the restriction.

Other companies were in the process of restarting fields. Royal Dutch Shell’s Shearwater platform was closed as of Thursday morning and expected to restart later in the day, a company spokesman said.

Forties is the biggest of the five North Sea crude grades underpinning dated Brent, a benchmark used for oil trading in Europe, the Middle East, Africa and Asia, and Brent futures.

The pipeline, which pumps about 450,000 bpd of oil and about a third of Britain’s offshore gas output, was previously shut in mid-December for about three weeks when a crack was discovered.

Oil traders expressed surprise at the latest outage, comparing the normally reliable oil flows in the North Sea to those in more volatile producing regions.